Here‘s a link on how to use several social media tool for formative assessment.
I personally think that the example of how to use Instagram to assess students on math theorem is interesting. The author stated that
A math teacher halts a lecture, projects incomplete math theorems on the board, and then assigns a different problem to each of several small groups. The twist is that the students post a photo of their written progress to Instagram every 90 seconds so that the instructor can monitor responses in real time.
I also agree that the hashtags feature on Twitter enable instant formative assessment. The author also describe how moderator can create Tweet Chat and discuss specific topic. The discussion can be captured and assessed by teachers.
Do you have other idea of how to use social media for formative assessment?